Journal
PHYSICAL CHEMISTRY CHEMICAL PHYSICS
Volume 17, Issue 12, Pages 8248-8257Publisher
ROYAL SOC CHEMISTRY
DOI: 10.1039/c4cp05943c
Keywords
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Funding
- Advanced Low Carbon Technology Research and Development Program (ALCA) of the Japan Science and Technology Agency (JST)
- Technology Research Grant Program of the New Energy and Industrial Technology Development Organization (NEDO) of Japan
- JSPS KAKENHI [26-165]
- Grants-in-Aid for Scientific Research [14J04098] Funding Source: KAKEN
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Certain molten complexes of Li salts and solvents can be regarded as ionic liquids. In this study, the local structure of Li+ ions in equimolar mixtures ([Li(glyme)] X) of glymes (G3: triglyme and G4: tetraglyme) and Li salts (LiX: lithium bis(trifluoromethanesulfonyl) amide (Li[TFSA]), lithium bis(pentafluoroethanesulfonyl) amide (Li[BETI]), lithium trifluoromethanesulfonate (Li[OTf]), LiBF4, LiClO4, LiNO3, and lithium trifluoroacetate (Li[TFA])) was investigated to discriminate between solvate ionic liquids and concentrated solutions. Raman spectra and ab initio molecular orbital calculations have shown that the glyme molecules adopt a crown-ether like conformation to form a monomeric [Li(glyme)](+) in the molten state. Further, Raman spectroscopic analysis allowed us to estimate the fraction of the free glyme in [Li(glyme)] X. The amount of free glyme was estimated to be a few percent in [Li(glyme)] X with perfluorosulfonylamide type anions, and thereby could be regarded as solvate ionic liquids. Other equimolar mixtures of [Li(glyme)] X were found to contain a considerable amount of free glyme, and they were categorized as traditional concentrated solutions. The activity of Li+ in the glyme-Li salt mixtures was also evaluated by measuring the electrode potential of Li/Li+ as a function of concentration, by using concentration cells against a reference electrode. At a higher concentration of Li salt, the amount of free glyme diminishes and affects the electrode reaction, leading to a drastic increase in the electrode potential. Unlike conventional electrolytes (dilute and concentrated solutions), the significantly high electrode potential found in the solvate ILs indicates that the solvation of Li+ by the glyme forms stable and discrete solvate ions ([Li(glyme)](+)) in the molten state. This anomalous Li+ solvation may have a great impact on the electrode reactions in Li batteries.
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